Cancel pending style request when loading style JSON (#3709)

Co-authored-by: Tim Sylvester <tim@wetdogweather.com>

include/mbgl/style/style.hpp

@@ -71,6 +71,8 @@ public:
     void addLayer(std::unique_ptr<Layer>, const std::optional<std::string>& beforeLayerID = std::nullopt);
     std::unique_ptr<Layer> removeLayer(const std::string& layerID);
 
+    void cancelPendingRequests();
+
     // Private implementation
     class Impl;
     const std::unique_ptr<Impl> impl;